Abrasive grit with high alumina content grain, in particular for application in applied and sintered abrasives, for example in scarfing grinders for slabs of steel alloy

ABSTRACT

An abrasive grit containing more than 96% by weight of sintered alumina, between 0.1 and 3% by weight of titanium oxide and between 0.1 and 3% by weight of manganese oxide, the sum of titanium oxide and manganese oxide remaining less than 4% by weight. The content by weight of titanium oxide and/or manganese oxide is preferably more than or equal to 0.5% and less than or equal to 1.3%.

TECHNICAL FIELD

The invention concerns the area of sintered abrasive grains used for the preparation of abrasive tools. It concerns more particularly the field of bonded and coated abrasives. Bonded abrasives are intended for the manufacture of products where the abrasive grains are dispersed in a resin-based bonding substance, typically grinding wheels. Coated abrasives are typically abrasive powders deposited on supports (paper, cloth, strip etc).

Even though it applies to all types of abrasive, the present invention is more particularly intended for the manufacture of bonded abrasives such as those used for making grinding wheels for the snagging of steel slabs, the deburring of raw cast parts or again for the regrinding of metals. These bonded abrasives will be made use of below for illustrating the invention.

STATE OF THE ART

Bonded and coated abrasives are prepared from abrasive grains, which may be obtained by either grinding a so-called “crude” product consisting of a solid product resulting from the solidification of a liquid product, for example an electrically fused abrasive such as corundum, or by the sintering of a powder consisting of grains whose composition and geometrical form promote good abrasive properties, or by using the “sol-gel” process, described for example in U.S. Pat. No. 4,574,003 (owner 3M).

Abrasive grains must possess good mechanical properties such as toughness or strength and good grinding action.

Strength is characteristic of the propensity of the grains to fracture in order to produce fragments under the effect of mechanical stress. The product being tested is calibrated according to the grade to be tested. After mechanical stress (tumbling in a drum filled with steel balls), a sample is screened through a column consisting of several screens of predetermined meshes. A specific coefficient is allocated to each recovered fraction, which makes it possible to classify its quality in terms of strength, expressed as a barycentric average of contents relating to each fraction (expressed as percentages). The greater the strength, the more closely the value obtained must approach the value of 100. An example is given at the end of the present document of the determination of the strength of a grade 12 grain (that is to say, one with an initial particle size of 1.7 to 2 mm).

It should be noted that hardness, which is a characteristic linked to plastic deformation of the material subjected to indentation, is a criterion, which is certainly necessary, but is not adequate for characterising the strength of an abrasive grain. There is, of course, a certain correlation, but there is no bijective relationship between the hardness of an abrasive grain and its abrasive properties; a very hard grain may be fragile and its rupture may have a beneficial effect (for instance the emergence of new sharp edges), but may also have negative effects (reduced grinding action, poor finish-imparting ability and the like). On the other hand, a softer grain may be less fragile, but may be better able to “regenerate its edges”.

The grinding action is the property of the grain to retain its cutting angles and also that of becoming fractured to produce fresh cutting angles. It may be characterised by a material removal power, which may be quantifiable by using, for instance, the ratio G=(quantity of material removed)/(quantity of abrasive used).

For bonded abrasive applications, certain conditions of use such as the snagging of steel alloy slabs may require the abrasive grains to confer on the bonded product a grinding action of the order of 200 to 2000 kg/hour under very high pressure (up to 55 daN/cm² under extreme conditions) coupled with a peripheral temperature potentially reaching up to 1000° C.

The range of abrasive products available on the market comprises

electrically fused white or brown corundum, but which has the drawback of being friable;

electrically fused zirconium corundum, which is tough, but whose hot grinding action is limited;

sintered bauxite, which has low grinding action, but makes possible a fine finish (due to the shape of the grains);

alumina obtained by the sol-gel process, which is too expensive for many applications;

sintered alumina, which does not possess acceptable strength and a grinding action, unless it has been produced at very high sintering temperatures.

To meet extreme conditions encountered when it is desired to remove material at a rate of the order of 200 to 2000 kg/h under very high pressure and temperature (of the order of 1000° C.), it is usual to employ grinding wheels based on zirconium corundum (typically containing 25% of ZrO₂) but such grinding wheels are not suitable for obtaining a good finish, so that it becomes necessary to employ a succession of several types of abrasive to accomplish a satisfactory snagging operation.

Statement of Problem

The applicant has accordingly sought to obtain abrasive grains possessing properties such as strength and grinding action, better than that possessed by products currently available on the market, with the exception of grains obtained by the sol-gel process, the target being that of approaching the properties of the latter, which possess good strength and grinding action, but whose cost of manufacture is prohibitive for many applications.

DESCRIPTION OF THE INVENTION

The object of the invention an abrasive grit essentially consisting of sintered alumina, containing over 96% by weight of alumina, between 0.1 and 3% by weight of titanium dioxide and between 0.1 and 3% of manganese oxide (expressed as MnO), the total of titanium dioxide and/or manganese dioxide being less than 4% by weight. The content of titanium dioxide+manganese oxide is preferably equal to or higher than 0.5% by weight. The content of titanium dioxide and/or manganese oxide is likewise preferably equal to or lower than 1.3% by weight.

The applicant has in fact found that by adding a small proportion of titanium dioxide and manganese oxide, the alumina could be sintered at a distinctly lower temperature and still retain its abrasive properties. Apart from a not negligible energy saving, the process according to the invention has the advantage of making possible the continuing use of existing furnaces, for example those previously used for the sintering of bauxite. On the other hand, the invention also makes possible the use of a low sintering temperature, which, in turn, makes possible the use of a continuous treatment furnace requiring a lower investment, being easy to control and able to deal with high throughputs. This may for example be a rotating furnace with a refractory lining and able to function continuously. Moreover, the duration of sintering is relatively short, of the order of 20 minutes for the plateau, which involves a complete cycle (the period of rise to sintering temperature+the period of sintering+the period of cooling) totalling between 30 and 120 minutes, typically 90 minutes from cold to cold.

Generally speaking, the addition of larger than negligible quantities of additives may result in said additives being precipitated in the vicinity of grain boundaries and result in a weakening of the abrasive product. This is why in practice the said addition is limited to contents of the order of 0.2% by weight (see high temperature sintered alumina in example 2 below, which contains 0.2% by weight of magnesia). In the context of the present invention, the applicant has found that surprisingly, the addition of titanium dioxide and magnesium oxide(s) in significant quantities of over 0.1% of each oxide makes it possible to maintain the performance of the product, regardless of the temperature at which it is used.

According to the invention, manganese oxide and titanium dioxide must be present in the sintered alumina powder to an extent of at least 0.1% by weight and not more than 3% by weight of each of the said types of oxide and not more than of a total of 4% by weight. The manganese oxide may be present as MnO or MnO₂, expressed as % by weight of MnO. In order to improve the effectiveness of sintering at low temperature, the weight content of titanium dioxide is preferably equal to or higher than 0.5% by weight and/or the weight content of manganese (expressed as MnO) is equal to or higher than 0.5% by weight. To limit the risk of reduction of the strength of the resulting abrasive product, the weight content of titanium dioxide is preferably equal to or lower than 1.3% by weight and/or the weight content of manganese oxides (expressed as MnO) is equal to or lower than 1.3% by weight. For the same reason, it is preferably endeavoured to limit the weight content of titanium dioxide and manganese oxide (expressed as MnO) to a value equal to or lower than 2.6% by weight

The weight content of manganese oxide is preferably almost equal to the weight content of titanium dioxide, with a typical ratio of between 0.8 and 1.2. The particles of alumina is preferably alpha alumina and advantageously originate from calcined alumina of an average diameter in the vicinity of 2 μm.

The grains obtained after sintering preferably consist of particles of alumina/manganese oxide(s)/titanium dioxide compounds of uniform size in the vicinity of 10 μm, typically between 5 and 20 μm.

Another object of the invention is a process for the manufacture of abrasive grit comprising the following successive steps, namely

a) preparing a mixture comprising:

a1) a powder consisting of grains of calcined alumina, whose crystallites have a mean diameter (expressed by D50) between 1 μm, preferably 1.5 μm, and 2.5 μm and typically 2 μm.

a2) a powder of titanium dioxide (TiO₂) added to the mixture to give a weight content of 0.1% to 3% and whose particle size D50 is in the vicinity of that of the alumina powder, for example between 1 and 3 μm, typically 2 μm.

a3) a powder of manganese oxide (MnO and/or MnO₂) added to the mixture to give a weight content of 0.1 to 3% by weight and whose particle size D50 is in the vicinity of that of alumina powder, for example between 1 and 3 μm, typically 2 μm,

the mixture being homogenised mechanically, typically by mixing.

b) shaping by pressure the resulting powder to obtain green shaped bodies,

c) drying of the green shaped bodies;

d) firing at a temperature below 1600° C.

The firing temperature is in the vicinity of 1500° C., typically between 1450° and 1550° C. The complete cycle (heating to sintering temperature, plateau at sintering temperature and cooling) is between 30 and 120 minutes. The furnace used is preferably a rotating continuous furnace.

The resulting abrasive grains are characterised by their density, their strength and their grinding action. Their micro-structure is controlled so as to obtain, following firing, uniform particle size of the alumina/manganese oxide(s)/titanium oxide compounds in the vicinity of 10 μm.

Advantageously, shaping of the powder under pressure is compacting green paste by extrusion, resulting in the formation of fibres, which are subsequently broken up in such a way as to yield quantities of green shaped bodies in the form of prisms of a given section and a given height.

Appendices

A1. The Effect of Sintering Temperature on the Properties of Sintered Alumina without an Addition of Manganese Oxides and Titanium Dioxide

Table I shows in respect of three calcined aluminas of different size gradings, values of strength resulting from sintering at 1600° and 1650° C. It is found that it is the alumina with the finest size grading, which possesses the best strength values, which nevertheless are still insufficient, the aim being that of exceeding 75. TABLE 1 Alumina A B C Median 4 2.1 0.4 diameter (μm) Sintering 1650 1600 1600 1650 1650 1600 temperature (° C.) Strength <20 <10 11 <20 <60 <50 A2 Comparison Between Different Products Available on the Market and a Product According to the Invention

Table 2 shows the values of strength measured on various products available on the market and in the last column, on a product according to the invention. The strength values correspond to averages by product, the individual values typically fluctuating by ±5 with respect to average values.

High temperature sintered alumina has a size grading similar to that of product C appearing above and includes a sintering agent of the magnesia type. It was sintered at a temperature in excess of 1600° C.

Sintered alumina with an addition of titanium dioxide and manganese oxide(s) according to the present invention comprises 1.2% by weight of TiO₂ and 1.2% by weight of manganese oxide expressed as MnO. It was sintered at 1500° C. It has a distinctly higher strength than that of any other sintered alumina, lower than that of sol-gel process alumina, but its cost is six times lower. It likewise has a strength lower than that of zirconium-based corundum, but is more suitable for obtaining finish grinding. TABLE 2 Zirconium Sintered Sintered corundum alumina (high Sol-gel Sintered alumina (low 25% ZrO2 temperature) alumina bauxite temperature) Selling 100 120 600 70 100 price (index) Density 4.25 3.85 3.94 3.84 3.85 (g/cm³) Strength 92 70 92 77 85 A3 Example of Determination of the Coefficient of Strength in Respect of Grade 12 Abrasive Grit

The basic sample is calibrated by selecting grains between 1.7 and 2 mm by successively screening through two screens (1.7 and 2 mm mesh)

It is ground by mechanical stress, typically by tumbling in a drum filled with steel balls. The column of screens used comprises the following screens defined by their respective meshes, namely, 1, 0.5, 0.25 and 0.125 mm.

Fractions of powder belonging to each of the following size categories are recovered

1 Ø_(grain)>1 mm

2 1 mm>Ø_(grain)>0.5 mm

3 0.5 mm>Ø_(grain)>0.25 mm

4 0.25 mm>Ø_(grain)>0.125 mm

5 0.125 mm>Ø_(grain)

If one calls Ti the relative weight of the fraction of powder recovered in the size category i [that is to say the ratio (mass of fraction i)/(initial mass of the sample before testing)], the strength is expressed by the equation Strength=4×T1+2×T2+T3+0.5×T4+0.25×T5

So that the initial powder shall correspond to an index of 100, the preceding value is divided by the sum of the weight coefficients. It is found that using this formula, the higher the quantity passing through the last screen (here Ø_(grain)<0.125 mm), the lower the value obtained; being very fragmented, the abrasive in question has generated a large quantity of “fines” and consequently has a lower strength value. 

1. An abrasive grit, essentially containing sintered alumina, comprising over 96% by weight of alumina, between 0.1 and 3% by weight of titanium dioxide and between 0.1 and 3% by weight of manganese oxide(s) (expressed as MnO), the total of titanium dioxide and manganese oxide(s) being less then 4% by weight.
 2. An abrasive grit according to claim 1, wherein the weight content of titanium dioxide is equal to or higher than 0.5% by weight.
 3. An abrasive grit according to claim 1, wherein the weight content of manganese oxide(s) (expressed as MnO) is equal to or higher than 0.5% by weight.
 4. An abrasive grit according to claim 1, wherein the weight content of titanium dioxide is equal to or lower than 1.3% by weight.
 5. An abrasive grit according to claim 1, wherein the weight content of manganese oxide(s) (expressed as MnO) is equal to or lower than 1.3% by weight.
 6. An abrasive grit according to claim 1, wherein the ratio of the weight content of manganese oxide to the weight content of titanium dioxide is between 0.8 and 1.2% by weight.
 7. An abrasive grit according to claim 1, wherein the particles after sintering of the alumina/manganese oxide(s)/titanium dioxide compounds are of a uniform size in the vicinity of 10 μm and typically between 5 and 20 μm.
 8. A process of manufacture of abrasive grit comprising the steps of: a) preparing a mixture comprising a1) a powder of calcined alumina grains whose crystallites have a mean diameter (expressed by D50) between 1 μm, preferably 1.5 μm and 2.5 μm, typically 2 μm; a2) a powder of titanium dioxide (TiO₂) added to the mixture to produce a weight content between 0.1 and 3% by weight and the particle size D50 whose particles is in the vicinity of that of the alumina powder, for example between 0.1 and 3 μm, typically 2 μm; a3) a powder of manganese oxide (MnO and/or MnO₂) added to the mixture to produce a weight content between 0.1 and 3% by weight and having a particle size D50 in the vicinity of that of the alumina powder, for example between 1 and 3 μm, typically 2 μm, the alumina+titanium dioxide+manganese oxide(s) mixture being homogenised mechanically, typically by mixing; b) shaping under pressure the powder thus prepared to obtain green shaped bodies; c) drying the green shaped bodies; and d) firing the dried, green shaped bodies at a temperature below 1600° C.
 9. A process according to claim 8 wherein the firing temperature is in the vicinity of 1500° C. and typically between 1450° and 1550° C.
 10. A process according to claim 8, wherein the complete cycle (heating to sintering temperature+plateau of sintering temperature+cooling) is between 30 and 120 minutes, typically 90 minutes from cold to cold.
 11. A process according to claim 8, wherein the furnace used is a continuous rotating furnace.
 12. A process according to claim 8, wherein shaping under pressure of stage b) is compacting green paste by extrusion, resulting in the formation of fibres that are subsequently broken up in such a way that it is possible to obtain quantities of green shaped bodies in the form of prisms of a given section and of a given height.
 13. A bonded abrasive product such as a grinding wheel, typically intended for snagging of steel slabs, comprising an abrasive grit according to claim
 1. 14. A coated abrasive product such as abrasive paper or cloth, comprising an abrasive grit according to claim
 1. 